Vercel React Best Practices

Comprehensive performance optimization guide for React and Next.js applications, maintained by Vercel. Contains 45 rules across 8 categories, prioritized by impact to guide automated refactoring and code generation.

When to Apply

Reference these guidelines when:

• Writing new React components or Next.js pages

• Implementing data fetching (client or server-side)

• Reviewing code for performance issues

• Refactoring existing React/Next.js code

• Optimizing bundle size or load times

Rule Categories by Priority

Priority Category Impact Prefix

1 Eliminating Waterfalls CRITICAL async-

2 Bundle Size Optimization CRITICAL bundle-

3 Server-Side Performance HIGH server-

4 Client-Side Data Fetching MEDIUM-HIGH client-

5 Re-render Optimization MEDIUM rerender-

6 Rendering Performance MEDIUM rendering-

7 JavaScript Performance LOW-MEDIUM js-

8 Advanced Patterns LOW advanced-

Quick Reference

1. Eliminating Waterfalls (CRITICAL)

• async-defer-await

• Move await into branches where actually used

• async-parallel

• Use Promise.all() for independent operations

• async-dependencies

• Use better-all for partial dependencies

• async-api-routes

• Start promises early, await late in API routes

• async-suspense-boundaries

• Use Suspense to stream content

2. Bundle Size Optimization (CRITICAL)

• bundle-barrel-imports

• Import directly, avoid barrel files

• bundle-dynamic-imports

• Use next/dynamic for heavy components

• bundle-defer-third-party

• Load analytics/logging after hydration

• bundle-conditional

• Load modules only when feature is activated

• bundle-preload

• Preload on hover/focus for perceived speed

3. Server-Side Performance (HIGH)

• server-cache-react

• Use React.cache() for per-request deduplication

• server-cache-lru

• Use LRU cache for cross-request caching

• server-serialization

• Minimize data passed to client components

• server-parallel-fetching

• Restructure components to parallelize fetches

• server-after-nonblocking

• Use after() for non-blocking operations

4. Client-Side Data Fetching (MEDIUM-HIGH)

• client-swr-dedup

• Use SWR for automatic request deduplication

• client-event-listeners

• Deduplicate global event listeners

5. Re-render Optimization (MEDIUM)

• rerender-defer-reads

• Don't subscribe to state only used in callbacks

• rerender-memo

• Extract expensive work into memoized components

• rerender-dependencies

• Use primitive dependencies in effects

• rerender-derived-state

• Subscribe to derived booleans, not raw values

• rerender-functional-setstate

• Use functional setState for stable callbacks

• rerender-lazy-state-init

• Pass function to useState for expensive values

• rerender-transitions

• Use startTransition for non-urgent updates

6. Rendering Performance (MEDIUM)

• rendering-animate-svg-wrapper

• Animate div wrapper, not SVG element

• rendering-content-visibility

• Use content-visibility for long lists

• rendering-hoist-jsx

• Extract static JSX outside components

• rendering-svg-precision

• Reduce SVG coordinate precision

• rendering-hydration-no-flicker

• Use inline script for client-only data

• rendering-activity

• Use Activity component for show/hide

• rendering-conditional-render

• Use ternary, not && for conditionals

7. JavaScript Performance (LOW-MEDIUM)

• js-batch-dom-css

• Group CSS changes via classes or cssText

• js-index-maps

• Build Map for repeated lookups

• js-cache-property-access

• Cache object properties in loops

• js-cache-function-results

• Cache function results in module-level Map

• js-cache-storage

• Cache localStorage/sessionStorage reads

• js-combine-iterations

• Combine multiple filter/map into one loop

• js-length-check-first

• Check array length before expensive comparison

• js-early-exit

• Return early from functions

• js-hoist-regexp

• Hoist RegExp creation outside loops

• js-min-max-loop

• Use loop for min/max instead of sort

• js-set-map-lookups

• Use Set/Map for O(1) lookups

• js-tosorted-immutable

• Use toSorted() for immutability

8. Advanced Patterns (LOW)

• advanced-event-handler-refs

• Store event handlers in refs

• advanced-use-latest

• useLatest for stable callback refs

How to Use

For detailed explanations and code examples, read the full compiled document: AGENTS.md

Each rule contains:

• Brief explanation of why it matters

• Incorrect code example with explanation

• Correct code example with explanation

• Additional context and references

Key Examples

Promise.all for Independent Operations (CRITICAL)

// ❌ Sequential: 3 round trips const user = await fetchUser() const posts = await fetchPosts() const comments = await fetchComments()

// ✅ Parallel: 1 round trip const [user, posts, comments] = await Promise.all([ fetchUser(), fetchPosts(), fetchComments() ])

Avoid Barrel File Imports (CRITICAL)

// ❌ Imports entire library (200-800ms import cost) import { Check, X, Menu } from 'lucide-react'

// ✅ Imports only what you need import Check from 'lucide-react/dist/esm/icons/check' import X from 'lucide-react/dist/esm/icons/x'

Dynamic Imports for Heavy Components (CRITICAL)

// ❌ Monaco bundles with main chunk ~300KB import { MonacoEditor } from './monaco-editor'

// ✅ Monaco loads on demand import dynamic from 'next/dynamic' const MonacoEditor = dynamic( () => import('./monaco-editor').then(m => m.MonacoEditor), { ssr: false } )

Use Functional setState (MEDIUM)

// ❌ Requires state as dependency, stale closure risk const addItems = useCallback((newItems) => { setItems([...items, ...newItems]) }, [items])

// ✅ Stable callback, no stale closures const addItems = useCallback((newItems) => { setItems(curr => [...curr, ...newItems]) }, [])

Constraints

Required Rules (MUST)

• Eliminate waterfalls: Use Promise.all and Suspense

• Optimize bundles: No barrel imports, use dynamic imports

• RSC boundaries: Serialize only the necessary data

Prohibited Patterns (MUST NOT)

• Sequential await: Do not execute independent fetches sequentially

• Array mutations: Use toSorted() instead of sort()

• Inline objects in React.cache: Causes cache misses

References

• React Documentation

• Next.js Documentation

• SWR

• better-all

• Vercel Blog: Optimizing Package Imports

• Vercel Blog: Dashboard Performance

Metadata

Version

• Current Version: 1.0.0

• Last Updated: 2026-01-22

• Compatible Platforms: Claude, ChatGPT, Gemini

• Original Source: vercel/agent-skills

Related Skills

• performance-optimization: General performance optimization

• state-management: State management

Tags

#React #Next.js #performance #optimization #vercel #waterfalls #bundle-size #RSC #frontend